Computer terminals within a system or organization are generally interconnected by some type of network, which allows data to be transmitted and received between the many elements (i.e. computer terminals, main frame, printers, etc.) of the network to communicate with each other. One particular type of network is a fiber distribution data interface or FDDI, which uses fiber optic cables and transceivers to create a network of computer terminals, printers and a mainframe. There are two types of FDDI cables, single-mode and multi-mode. As will be explained more specifically below, single-mode FDDI cable networks are more expensive and are generally used for long distance communication, usually for distances between 2 and 20 kilometers, and multi-mode FDDI cable networks are used for short distance communication, usually less than 2 Km.
FIGS. 1-3 illustrate a typical single-mode FDDI cable. Referring now to FIG. 1, a single-mode cable 20 is shown which includes optic fibers 22 and 23 surrounded by aramid yarns 30 for strength and an outer jacket 21 for protection. Cable 20 has a first end 37 and a second end 38. First end 37 of cable 20 is connected to a single-mode fixed shroud duplex transceiver connector 39. As is well known in the art, connector 39 is "keyed" or connectorized by elements 24. A single-mode communication transceiver (not shown) of FDDI node 1 is receptively "keyed" or connectorized such that the transceiver of node 1 will only allow the connection of a single-mode cable connector. The single-mode connectorization between the transceiver of a node and the connector of a cable is a well-known ANSI standard. Single-mode cable 20 is mounted in connector 39 so that optic fiber 22 is optically coupled through fiber termination 26 of connector 39 to a laser diode (not shown) that is in the communication transceiver of node 1 and optic fiber 23 is optically coupled through fiber termination 28 of connector 39 to a photodetector (not shown) that is in the communication transceiver of node 1.
A second end 38 of single-mode cable 20 is connected to a single-mode fixed shroud duplex transceiver connector 40, which is "keyed" or connectorized by elements 25. A single-mode communication transceiver (not shown) of FDDI node 2 is receptively "keyed" or connectorized such that the communication transceiver of FDDI node 2 will only allow the connection of a single-mode cable connector. The second end 38 of cable 20 is coupled to connector 40 such that optic fiber 22 is optically coupled through a fiber termination 29 of connector 40 to a photodetector (not shown) in the transceiver of node 2 and optic fiber 23 is optically coupled through a fiber termination 27 of connector 40 to a laser diode transmitter (not shown) in the transceiver of node 2.
During a data transfer from node 1 to node 2, a laser diode in the transceiver of node 1 emits a laser beam signal that is transmitted into the optic fiber 22 at end 26 of connector 39. The laser beam signal then propagates through optic fiber 22 and is received at the other end at fiber termination 29 of connector 40, which couples the laser beam signal to a photodetector in the transceiver of node 2. The signal is then converted by opto-electronic circuitry (not shown) in node 2's transceiver to a form that can be "read" by node 2. In a similar data transfer from node 2 to node 1, a laser in the transceiver of node 2 emits a laser beam signal that is transmitted into optic fiber 23 via fiber end 27 of connector 40. The laser beam signal then propagates through optic fiber 23 and is received at the other end at fiber termination 28 of connector 39, which couples the laser beam signal to a photodetector in the transceiver of node 1. The signal is then converted by opto-electronic circuitry (not shown) in node 1's transceiver to a form that can be "read" by node 1.
FIG. 2 shows a lateral cut-away view of single-mode cable 20 and FIG. 3 shows a longitudinal cut-away view of a short segment of single-mode cable 20. Referring now to FIGS. 2 and 3, cable 20 includes two optical fibers 22 and 23, which are made up of fiber optic cores 32 and 36 surrounded by optical cladding layers 65 and 66 and mechanically strippable plastic layers 31 and 35, respectively. The fiber optic cores 32 and 36 are each 9 microns in diameter. Optic fibers 22 and 23 are surrounded by aramid yarns 30 for strength and an outer jacket 21 for protection. Optic fibers 22 and 23 transmit coherent light, such as light 33 emitted from laser diode 34. Single-mode fiber optic cables, such as cable 20, are capable of conducting coherent light signals up to about 16 or 20 kilometers, and therefore are used for transmitting data between remote FDDI nodes. Technically, single-mode fiber optic cables could also be used for close range transmission of data, such as between computers within a single office. However, the high cost of single-mode communication transceivers (approximately $1,200.00) generally prohibits such use.
FIGS. 4-6 illustrate a typical multi-mode FDDI optic cable 41. Referring now to FIG. 4, a multi-mode cable 41 is shown which includes optic fibers 43 and 44 surrounded by aramid yarns 53 for strength and an outer jacket 42 for protection. Cable 41 has a first end 54 and a second end 55. The first end 54 of cable 41 is mounted in a multi-mode fixed shroud duplex transceiver connector 52. As is well known in the art, multi-mode connector 52 is "keyed" or connectorized by elements 45. A multi-mode communication transceiver (not shown) of FDDI node 3 is receptively "keyed" or connectorized such that the transceiver of node 3 will only allow the connection of a multi-mode cable connector. The multi-mode connectorization between the transceiver of a node and the connector of a cable is a well-known ANSI standard.
Multi-mode cable 41 is mounted in connector 52 in such a manner that optic fiber 43 is optically coupled through fiber termination 47 of connector 52 to a light emitting diode (LED) (not shown) that is in the communication transceiver of node 3 and optic fiber 44 is optically coupled through fiber termination 49 of connector 52 to a photodetector (not shown) that is in the communication transceiver of node 3.
A second end 55 of multi-mode cable 41 is mounted in a multi-mode fixed shroud duplex transceiver connector 51, which is "keyed" or connectorized by elements 46. A multi-mode communication transceiver (not shown) of FDDI node 4 is receptively "keyed" or connectorized such that the communication transceiver of FDDI node 4 will only allow the connection of a multi-mode cable connector. The second end 55 of cable 41 is mounted in connector 51 in such a manner that optic fiber 43 is optically coupled through fiber termination 50 of connector 51 to a photodetector (not shown) in the transceiver of node 4 and optic fiber 44 is optically coupled through fiber termination 48 of connector 51 to a light emitting diode (LED) (not shown) in the transceiver of node 4.
During a data transfer from node 3 to node 4, the LED (not shown) in the transceiver of node 3 emits a light beam signal that is transmitted to optic fiber 43 via fiber termination 47 of connector 52. The light beam signal then propagates through optic fiber 43 and is received at the opposite end of optic fiber 43 at fiber termination 50 of connector 51, which couples the light beam signal to the photodetector (not shown) in the transceiver of node 4. The light signal is then converted by opto-electronic circuitry (not shown) in node 4's transceiver to a form that can be "read" by node 4. In a similar data transfer from node 4 to node 3, the LED in the transceiver of node 4 emits a light beam signal that is transmitted through into optic fiber 44 at fiber termination 48 of connector 51. The light beam signal then propagates through optic fiber 44 and is received at the opposite end of optic fiber 44 at fiber termination 49 of connector 52, which couples the light beam signal to the photodetector (not shown) in the transceiver of node 3. The light signal is then converted by opto-electronic circuitry (not shown) in node 3's transceiver to a form that can be "read" by node 3.
FIG. 5 shows a lateral cut-away view of multi-mode cable 41 and FIG. 6 shows a longitudinal cut-away view of a short segment of multi-mode cable 41. Referring now to FIGS. 5 and 6, cable 41 includes two optic fibers 43 and 44 that are made up of fiber optic cores 62 and 63, which are surrounded by optical cladding layers 56 and 57 and mechanically strippable plastic layers 58 and 59, respectively. Optic fibers 43 and 44 are surrounded by aramid yarns 53 for strength and an outer jacket 42 for protection.
The fiber optic cores 62 and 63, through which the light travels, are each 62.5 microns in diameter and are made up of many concentric layers of glass with different indexes of refraction, higher indexes towards the center, lower indexes toward the outside. As is well known in the art, light travels faster in a lower index of refraction material. The light entering a multi-mode optic fiber with graded-index of refraction at angles other than zero end up taking a longer path than light that enters at an angle close to zero. Therefore, the light that takes a longer path spends more of its time in the faster lower indexes of refraction glass and arrives closer to the same time as the light that traveled straight through. These properties of the multi-mode fiber help reduce multi-path dispersion or pulse spreading that occurs with non-coherent light sources.
Optic fibers 43 and 44 conduct non-coherent light, such as light emitted from a light emitting diode 60. Therefore, because of the pulse spreading, multi-mode fiber optic cables are only capable of transmitting light signals a distance of up to about 2 kilometers. For this reason, multi-mode cables are used for transmitting data signals between local FDDI nodes, such as nodes within a single office building or within a campus or office park environment. Most FDDI system owners/managers would prefer to use multi-mode communication paths whenever possible, since multi-mode transceivers, at approximately $150.00, are much less expensive than single-mode transceivers, at approximately $1,200.00.
Given the above overview of multi-mode and single-mode FDDI communication systems, it will be readily apparent that the inexpensive multi-mode communication system is generally used whenever possible for communications between FDDI nodes. However, when the distance between FDDI nodes exceeds 2 kilometers, the data transfers must be accomplished via a single-mode FDDI communication system.
FIG. 7 shows a schematic diagram of a typical FDDI network 69 which has a local site 70 with nodes 10, 11, 12, and 13 and a distant site 71 with nodes 14, 15, 16, and 17. As will be noted by reference to FIG. 7, each FDDI node 11 to 17 has an A transceiver and a B transceiver. An A transceiver of a first node must be connected to media compatible type B transceiver of a second node, and vice versa. For example, local node 10 of network 69 has a single-mode (S) transceiver A that is connected via a 2-20 kilometer single-mode (S) cable 72 to a single-mode (S) B transceiver of distant node 17. Similarly, node 10 has a multi-mode (M) B transceiver that is connected by a multi-mode cable (M) 73 of 2 kilometers or less to a multi-mode (M) A transceiver of local node 11. Likewise, each node in network 69 is connected to two other nodes by FDDI cables 72 to 79 with the mode (i.e., single-mode (S) or multi-mode(M)) of the transceivers and connecting cables being determined by the distance between the nodes being connected (i.e., less than or greater than 2 kilometers). Although it is possible to change the transceivers of a node from a multi-mode transceiver to a single-mode transceiver, or vice versa, for cost reasons, the transceivers A and B of a node are generally permanent.
It should be noted that a laser source is incompatible with multi-mode fiber, because too much optical energy would be coupled into the fiber which would saturate or overdrive the photodetector at the other end of the fiber. Moreover, an LED source is incompatible with single-mode fiber, because too little optical energy would be coupled into the fiber and the signal at the other end would be too weak for the photodetector to receive. In addition, the optical fiber used with an LED source must be constructed with concentric layers of glass with a graded index of refraction in order to reduce multi-path dispersion or pulse stretching. For these reasons, laser sources must launch their signal into a single-mode fiber and LED sources must launch their signal into a multi-mode fiber. Accordingly, transceivers and cable connectors are connectorized or "keyed" so that only a single-mode FDDI cable may be coupled to a laser transceiver and only a multi-mode FDDI cable may be coupled to an LED transceiver. Once a node's transceiver is established as either multi-mode or single-mode, it is also established that the node can only communicate through that transceiver with a node having a similar type transceiver.
The primary problem with FDDI nodes having preset, permanent transceivers and only being able to communicate with like-mode transceivers is just that--a node's single-mode transceiver cannot be directly connected to a multi-mode transceiver of a second node, and vice versa. This problem may arise when a network analyzer (also known as a network advisor or a protocol analyzer) has to be inserted into the network ring. For example, if a network analyzer was inserted into the ring between nodes 10 and 17 to analyze the communications between the two nodes, the connection would require a second single-mode cable and the network analyzer would need to have two single-mode transceivers, because node 10 and 17 are communicating with each other via a single-mode communication media. However, if the network analyzer was then inserted into the ring between nodes 10 and 11 to analyze the communications between those two nodes, the connection would require a second multi-mode cable and the network analyzer would need to have two multi-mode transceivers, because nodes 10 and 11 are communicating with each other via a multi-mode communication media.
The present situation is costly; in order to be capable of analyzing a FDDI network ring along any given point, a network analyzer must be equipped with two single-mode transceivers, two multi-mode transceivers, a single-mode cable, and a multi-mode cable. As would be readily apparent, a network analyzer with four transceivers must also have additional circuitry to allow for switching between and enabling of the transceivers, etc. Clearly, such an analyzer is not only costly, but also bulky and not readily portable, which is a disadvantage as an analyzer must be transported to any location in a network that needs to be analyzed.
One solution that addresses the size and portability of a network analyzer, but not the cost has been the use of communication "pods", one for single-mode communication and one for multi-mode communication. Each pod contains two transceivers, both either single-mode or multi-mode. If a network technician knows that he will be testing a network between two single-mode nodes, he inserts a single-mode pod into the network analyzer and if he will be testing a network between two multi-mode nodes, he inserts a multi-mode pod. As stated previously, this solution addresses the overall size of the analyzer, but not the cost, since a network analyzer must still be equipped with four transceivers and two cables. Furthermore, situations may arise where there is a need for direct communication between a single-mode node and a multi-mode node in an FDDI communication network, such as relocation of nodes to different facilities, etc.
Accordingly, there is need in the field of fiber optics for a means of direct communication between single-mode FDDI nodes and multi-mode FDDI nodes. There is further need in the field for a solution to the expensive, redundant need for transceivers in network analyzers and other network inserts. The present invention meets these and other needs.